Infrared imaging device with photoconductive target



Aug. 20, 1958 G. A. MORTON ET Al. 3,398,316

INFRARED IMAGING DEVICE WITH PHOTOCONDUCTIVE TARGET Filed Aug. 4, 19553,398,316 v INFRARED IMAGING DEVICE WITH PHOTOCONDUCTIVE TARGET GeorgeA. Morton and Robert J. Pressley, Princeton, and Stanley V. Forgue,Cranbury, NJ., assignors, by mesne assignments, to the United States ofAmerica as represented bythe Secretary of the Army Filed Aug. 4, 1955,Ser. No. 526,514 3 Claims. (Cl. 315-10) This invention relates toimaging devices for the infrared portion of the light spectrum. Inparticular this in`- vention relates to a novel photoconductive target-'for use in a new improved infrared pickup tube.

In the past there have been various materials suggested for use astargets for infrared pickup tubes. Generally, it

nited States Patent() has lbeen necessary that the materials used fortargets in Y these devices have a resistivity of at least 1011 ohm cm.in order that the target have the ability to store electrical chargesfor the 1/30 second frame time of the scanning beam. For line storagetype of operation, materials having a resistivity of approximately 109ohm cms. are usable. A further requirement of the materials used ininfrared pickup devices is that these materials be highly sensitive atwavelengths that extend Well into the infrared portion of the lightspectrum. Some materials are known that are sensitive to infraredwavelengths of the desired frequency. However, these materials do nothave the required resistivity for pickup tube operation. Other knownmaterials have the proper resistivity but are not sensitive to thedesired infrared wavelengths.

It has been found that some materials have resistivities that increaseas the material is cooled. Therefore, certain materials which have aresponse that extends into the infrared range of the spectrum are usablewhen the target is cooled to a low temperature. In the pickup tubes usedfor infrared purposes prior to this invention, cooling of the target hasbeen a considerable problem. One complicating factor is that the targetreceives heat by radiation from the thermionic cathode of the electrongun used in the tube.

It is therefore an object of this invention to provide a new andimproved infrared pickup device.

It is another object of this invention to provide a new and novel targetelectrode for an infrared pick-up device.

It is a further object of this invention to provide a new and improvedinfrared pickup tube including therein a new and novel target electrode.

These and other objects are accomplished in accordance with thisinvention by providing a new and improved photoconductive target that issensitive to infrared radiations, and a novel tube structure is providedfor cooling the target. Generally, the tube comprises an electron gun,arranged in an off-axis arm of an evacuated envelope, for producing anelectron beam. The electron beam from the gun is deflected around a bendin the elongated envelope to enter the main portion thereof where it isscanned over the target structure. A return beam from the targetstructure is defiected into a second off-axis arm of the envelope tostrike an electron multiplier and produce output signals therefrom. Withthis arrangement of tube structures, the target of the tube is shieldedfrom the gun cathode so that heat radiated therefrom is prevented fromstriking the target. The improved target includes a sensitive infraredphotoconductor of the single crystal or polycrystalline type. The targetmaterial is suitably activated to provide a spectral sensitivitycorresponding to energies associated with the desired infrared wavelengths.

The above and other objects will be better understood by reference tothe following description when read in conjunction with the accompanyingsingle sheet of drawings wherein:

FIGURE 1 is a fragmentary sectional view of a pickup tube in accordancewith this invention;

FIGURE 2 is an enlarged fragmentary sectional view of the target shownin FIGURE l.

V ReferringA specifically now to FIGURE l, there is shown a fragmentarysectional view of a pickup tube 10 in accordance with this invention.The tube 10 comprises an envelope 12 having a main, or body, portion 14and two off-axis arms 16 and 17, which arms have axes offset from theaxis of the body portion. As shown, the envelope 12 also includes athird arm 18, the purpose of which will be explained hereinafter. Withinthe arm 16 there is provided an electron gun 20 that includes aconventional cathode 21, control electrode 26 and one or moreaccelerating electrodes 22 for providing an electron beam 23.Surrounding the end of electron gun 20 is a first hollow tubularaccelerating electrode 28. Adjacent to the other end of acceleratingelectrode 28 is second accelerating eletrode 28 for accelerating thebeam 23 down the arm 16 into the body portion 14 of envelope 12. Theelectron gun 20 may be of any conventional type and further descriptionthereof is not deemed necessary. Surrounding the arm 16 is a focus coil24 for providing a magnet field parallel to the axis of envelope portion16 and for focusin-g the electron beam 23 as it traverses the arm 16.Surrounding the focus coil 24 is an alignment coil 25 for providing afield transverse to the axis of envelope portion 16 and for correctingany misalignment of the electron beam 23. Both the focus coil 24 and thealignment coil 25 may be of any well known type and therefore it is notbelieved to be necessary to describe them in detail.

In the junction area of the arms and body portion of envelope 12 thereis provided a decelerating field provided by electrode 31 which isconnected to a conductive coating 33 such as tin oxide. Electrode 31 isoperated at a potential that is negative to accelerating electrode 28 todecelerate the beam as it enters the junction region of the envelope, aswell as to aid in maintaining beam focus. The electron beam 23 is bentinto alignment with the body portion 14 of envelope 12 by means of ainagnetic field produced by an electromagnet producing field linesperpendicular to the path of the electron beam of the magnet 27. Onlyone pole of the magnet is shown for simplicity of illustration with theother pole piece being arranged on the opposite side of envelope 12.When the electron beam 23 passes through the magnetic field produced bythe electromagnet 27, the path of the electrons in the beam is bent toenter the body portion 14 of envelope 12. The current in theelectromagnet 27 is adjusted to give optimum deflection, or bending, ofthe beam into the body portion of envelope 12. The deflection eld usedhere is normally a few gauss in magnitude.

Within the body portion 14 of the envelope 12 there is provided a hollowtubular focusing electrode 34, which forms an electron lens with theconductive coating 33, and through which the electron beam travels in arelatively field free space. Adjacent to the target end of the tubularelectrode 34 is a hollow decelerating electrode 36 that is closed on oneend by an apertured mesh screen 38. The mesh screen 38 is permeable tothe electron beam 23 and acts as a decelerator electrode in certaintypes of operation.

Surrounding the body portion 14 of envelope 12 is a deliecting yoke 29which, in turn, is surrounded by a main focusing coil 30 providing amagnetic field parallel to the axis of envelope portion 14. When theelectron beam 23 from gun 20 is deflected into the body portion 14 ofthe envelope 12, it enters the body portion 14 substantially on the axisthereof, and substantially follows the magnetic lines of force of thefocus coil 30 which focuses the beam 23 onto a target 32 in the otherend of the envelope. Potentials are applied to the deflection yoke 29 todeect the beam over the surface of target 32, in a well known manner.

The target 32 may comprise layer 42 of photoconductive material, and hasa conductive backing, or signal plate 40 formed of a very thin layer ofevaporated gold. The conductive coating 40 should be thick enough to beelectrically conductive, yet thin enough to be transparent to infraredradiations. The thickness of the gold coating 40 is between 200 to 400angstrom units in thickness.

In accordance with this invention, the photoconductive material 42 iseither a single crystal, or a polycrystalline material into whichimpurity materials have been introduced to provide photoconductivesensitivities in the desired range of the spectrum and the properresistivity for use in a pickup tube. The crystalline target materialmay be doped, i.e., it may have impurities introduced into the materialby any of the recognized procedures.

A target material in accordance with this invention that has producedexcellent results is one obtained from a single crystal of silicon thatis doubly doped with gold and indium. A crystal of this type is formedby adding sufficient material to a crucible, not shown, in which thematerials are melted, to provide gold atoms of a concentration which iswithin the approximate range of one half the number of indium atoms to asubstantially equal number of gold atoms as indium atoms in theresulting silicon crystal. With the presently available silicon, thetotal amount of impurity atoms in the iinal crystal is approximately1015 to 1016 impurities per cubic centimeter of the crystal.

Another target material in accordance with an embodiment of thisinvention is a crystal of doubly doped silicon wherein the dopingmaterials are boron and gold. These materials are added in amounts toprovide a concentration of gold that is within the approximate range ofone half the number of boron atoms to substantially equal concentrationof gold and boron atoms in the final crystal of silicon.

A still further target material in accordance with this invention is acrystal of germanium that is doped with atoms of arsenic and gold. Theamount of gold atoms added is within the approximate range of one halfthe number of atoms of arsenic to a substantially equal concentration ofgold and arsenic atoms in the nal crystal.

Specific examples of the proportions of the materials in the embodimentsof the target 32 that were described above are as follows. (l) To 100grams of silicon add 10 grams of gold and 0.14 gram of indium, (2) To100 grams of silicon add l microgram of boron and 2.5 grams Iof gold,(3) To 100 grams of germanium add 450 miligrams of gold, to this mixtureadd 500 grams of germanium doped with arsenic at a concentration ofapproximately micrograms of arsenic per cubic centimeter of germanium.

The target 32 may be varied in thickness from about two to thirty mils.The most sensitive targets made to date were under ten mils in totalthickness. Diameters to about one inch have been used.

Any of the targets described above may be treated with a non-reflectingmaterial to reduce the radiation lost by reflection at the radiationincident surface of the target. An example of such a material is amixture of 32 grams of sodium hydroxide in 100 cubic centimeters ofwater. The solution is placed on the radiation incident side of target32 for live minutes at 75C. Then hydrolluoric acid is applied to thesame side for ve minutes at room temperature. This materialsubstantially reduces radiation lost by reflection, and also iselectrically conductive so that it may be used as the signal plate 40.

When desired, a very thin film of evaporated gold on the radiationincident side of target 32 may be used as a signal plate 40, as wasdescribed above.

Surrounding the target 32 is a target cooling cell that comprises ahollow target support member on which the target 32 is supported asshown. The target 32 is held in the hollow annular member 50 by a pairof screws 51 that press an annular member 53 against the target. Thetarget is pressed against a relatively soft indium washer 55 which isseated in the hollow support member 50. The hollow elongated supportmember 50, which may be of a material such as silver plated copper,encloses an annular space 56 through which a coolant, eg. liquidnitrogen, may be circulated to cool the target 32. The space 56communicates with the exterior of the cooling cell by means of are-entrant portion 54. The temperature of the target 32 may be measuredby means of a thermocouple 61 that contacts an edge of target 32. Withliquid nitrogen used as a coolant, targets have been operated attemperatures that were about 2 degrees above that of the liquidnitrogen, i.e. 77 K.

Adjacent to the other end of the elongated target support 50 issupported a thin, e.g. 1/16 of an inch, infrared transmitting window 52that may be of a material such as sapphire. The window 52 may beremovably positioned on the cooling cell by means of screws S7 thatpress the window against a gasket 65 on the cooling cell. In the areaaround both the window 52 and the target 32 the envelope is evacuatedsince, the target cooling cell is sealed to the body portion 14 ofenvelope 12 by means of a rubber gasket The reason for the targetcooling cell is that it has been found that certain materials operatewith greater sensitivity, and more useful resistivities, when thesematerials are cooled to particular temperatures. It should be understoodthat the target 32 may be made of many known materials which haveinfrared sensitivity when cooled to a predetermined temperature.

With cooling below liquid nitrogen temperature, even longer wavelengthresponse can be obtained with the targets described above. At liquidhelium temperature one could use silicon doped only with indium at thedesired wavelengths while still maintaining a resistivity that issufficiently high for pickup tube operation. In this example thewavelength response corresponding to the indium impurities would extendto about 8 microns. Other impurities, e.g. gallium, having even longerwavelength response, could be used at liquid helium temperature.

During operation, a coolant is also circulated around the walls of thebody portion 14 by being inserted into the inlet 63 which permits thecoolant to flow around the body portion 14 of envelope 11, and betweenthe focusing coil 30 and the deflection yoke 29. An example of a coolantfor this purpose is air that has been cooled in a liquid nitrogen heatexchanger, not shown.

When the electron beam 23 is deflected into the body portion of envelope14, the heat from the cathode is not deflected and thus impinges on thewalls of body portion 14. Thus, the heat from the cathode is cooled bythe coolant inserted into inlet 63 so that no heat from the cathode 21reaches the target.

Enclosed within the arm 17 of envelope 12 are three hollow tubularaccelerator electrodes 43, 44 and 46 for accelerating a return electronbeam 45 into a conventional electron multiplier 47 from which outputsignals are obtained during tube operation. The return beam 4S isdeflected into arm 17 by means of the electro magnet 27, due to the factthat it is traversing the magnetic fields from the magnet in theopposite direction from that of scanning beam 23, and is focused ontothe electron multiplier by a focusing coil 58.

When potentials are applied to the tube 10, such as those shown as anexample in FIGURE 1, the beam 23 scans the target 32 at energies suchthat more primary electrons land on the target than secondary electronsare driven from the target. Thus, when no radiation is directed onto thetarget 32, the scanned surface of the photoconductor 42 is driven in anegative direction to a reference potential that is substantially thesame as that of cathode 21. When infrared radiations are directed ontothe photoconductor 42, it becomes conductive in the elemental areas thatare struck by the radiation to establish a charge, with respect to thereference potential, on the scanned surface of the photoconductor 42.When the beam 23 re-scans the charged areas, it replaces the referencepotential. This replacement of the reference potential produces avariation in the return beam 4S which corresponds to the amount ofradiation on the target. The variation of the return beam 45 acts as anoutput signal from tube 10, when this variation is multiplied by themultiplier 47.

Arm 18 may be used for (l) inserting images onto the scanned side oftarget 32, or (2) a target may be tested by inserting another electrongun, not shown, in the arm to determine how a target is affected by theheat from a straight through gun as compared to the off-axis gun 20.Likewise, thermal radiation characteristics of certain electron guns maybe evaluated in this manner.

This invention has described novel target structures as well as a noveltube structure. The targets are extremely sensitive to infraredradiation, as an example, they have shown good response beyond 3.7microns. The novel tube structure provides an excellent means forcooling the target during operation.

What is claimed is:

1. An infrared pickup tube comprising an envelope including an elongatedbody portion having an axis and a pair of off-axis arms on one end ofsaid body portion, said arms having axes which intersect the axis of thebody portion at an angle, an infrared sensitive photoconductive targetin the other end of said body portion, an electron gun positioned insideand adjacent the outer end of one of said arms at a point sufficientlyremote from the intersection of the body portion and the arm that astraight line contained within the tube cannot interconnect the electrongun and the target because of the offset of the arm axis, said electrongun being positioned to emit an electron beam substantially along theaxis of the arm containing the gun, an electron multiplier in t-he otherof said arms, and means adjacent to the intersection of said bodyportion and said arms for bending the emitted electron beam from theaxis of the arm containing the electron gun to the axis of the bodyportion for impingement on said target and for bending an electron beamreflected from said target along the axis of the body portion to theaxis of the arm having the electron multiplier.

2. An infrared pickup tube comprising an envelope including an elongatedbody portion having an axis and a pair of off-axis arms on one end ofsaid body portion, said arms having axes which intersect the axis of thebody portion at an angle, an infrared sensitive photoconductive targetin the -other end of said body portion, said target comprising atransparent conductor, a crystal of silicon on said conductor and havingsubstantially the same number of atoms of impurities of boron as thenumber of atoms of impurties of gold in said silicon, an electron gunpositioned inside and adjacent the outer end of one of said arms at apoint where a straight line contained within the tube cannotinterconnect the electron gun and the target, said electron gun beingpositioned to emit an electron beam substantially along the axis of thearm containing the gun, an electron multiplier in the other of saidarms, magnetic means adjacent the intersection of said body and saidarms for bending the emitted electron beam from the axis of the armcontaining the electron gun along the axis of the body portion forimpingement on said target and for bending a reflected electron beamfrom the target along the axis of the body portion to the axis of thearm having the electron multiplier, and means for cooling said target.

3. An infrared pickup tube comprising an envelope having an elongatedbody portion having an axis and a pair of off-axis arms at one end ofsaid body portion, said arms having axes which intersect the axis of thebody portion at an angle, a target electrode adjacent to the other endof said body portion, said target electrode including a crystal ofsilicon having gold and indium irnpurities therein, said impuritiesbeing present at a concentration of approximately 1015 to 1016 impurityatoms per cubic centimeter of said crystal, said target also including atransparent conductive coating on one surface of Said crystal, aninfrared window at said other end of said body portion and spaced fromsaid target electrode, means for cooling said target electrode and thespace between said target and said window, means for cooling the `wallsof said body portion, an electron gun positioned inside and adjacent theouter end of one of said arms at a -point where a straight linecontained within said envelope cannot interconnect the electron gun andthe target, said electron gun being further positioned to emit anelectron beam substantially along the axis of the arm containing thegun, means adjacent to the intersection of said body portion and saidarms for bending the emitted electron beam from the axis of the armcontaining the electron gun along the axis of the body portion forimpingement on said target and for bending a reflected electron beamfrom the target along the axis of the body portion to the axis of thearm having the electron multiplier, and means for scanning said beamover said crystal.

References Cited UNITED STATES PATENTS 2,249,025 7/ 1941 Morton et al.313-65 2,522,153 9/1950 Andrews 313-32 2,524,035 10/1950 Bardeen et al.317-235 2,567,970 9/1951 Scal et al 317-235 2,597,285 5/1952 Pfann317-235 2,701,326 2/1955 Pfann et al. 317-235 2,213,177 8/1940 Iams313-67 2,648,794 8/ 1953 Henson 313-67 2,732,469 1/ 1956 Palmer 201-632,765,385 10/ 1956 Thomsen 201-63 2,374,914 5/1945 Behne et al 313-67OTHER REFERENCES Optical and Photoconductive Properties of Silicon andGermanium by E. Burstein, G. Picus, and N. Sclar. Presented at theConference on Photoconductivity, Atlantic City, NJ., November 4-6, 1954.Copy of paper in Photoconductivity Conference, 'published by John Wileyand Sons, Inc., New York, N.Y., pp. 353-413.

RODNEY D. BENNETT, Primary Examiner.

I. P. MORRIS, Assistant Examiner.

1. AN INFRARED PICKUP TUBE COMPRISING AN ENVELOPE INCLUDING AN ELONGATEDBODY PORTION HAVING AN AXIS AND A PAIR OF OFF-AXIS ARMS ON ONE END OFSAID BODY PORTION, SAID ARMS HAVING AXES WHICH INTERSECT THE AXIS OF THEBODY PORTION AT AN ANGLE, AN INFRARED SENSITIVE PHOTOCONDUCTIVE TARGETIN THE OTHER END OF SAID BODY PORTION, AN ELECTRON GUN POSITIONED INSIDEAND ADJACENT THE OUTER END OF ONE OF SAID ARMS AT A POINT SUFFICIENTLYREMOTE FROM THE INTERSECTION OF THE BODY PORTION AND THE ARM THAT ASTRAIGHT LINE CONTAINED WITHIN THE TUBE CANNOT INTERCONNECT THE ELECTRONGUN AND THE TARGET BECAUSE OF THE OFFSET OF THE ARM AXIS, SAID ELECTRONGUN BEING POSITIONED TO EMIT AN ELECTRON BEAM SUBSTANTIALLY ALONG THEAXIS OF THE ARM CONTAINING THE GUN, AN ELECTRON MULTIPLIER IN THE OTHEROF SAID ARMS, AND MEANS ADJACENT TO THE INTERSECTION OF SAID BODYPORTION AND SAID ARMS FOR BENDING THE EMITTED ELECTRON BEAM FROM THEAXIS OF THE ARM CONTAINING THE ELECTRON GUN TO THE AXIS OF THE BODYPORTION FOR IMPINGEMENT ON SAID TARGET AND FOR BENDING AN ELECTRON BEAMREFLECTED